This invention relates to a battery charging apparatus which charges a plurality of rechargeable batteries connected in series.
A battery charger, which charges a plurality of rechargeable batteries, connects those batteries in parallel for charging [refer to Japanese Patent Application 2002-298930]. As shown in FIG. 1, the battery charger cited in this disclosure connects switches 41 in series with each rechargeable battery 42, and those switches 41 are turned ON and OFF by a charging control section 43. A rechargeable battery 42 is charged when a switch is turned ON and charging is suspended when the switch is turned OFF. The charging control section 43 keeps a switch 410N until the rechargeable battery 42 has reached full charge, and turns the switch 41 OFF when the rechargeable battery 42 is fully charged. Since all the rechargeable batteries 42 do not become fully charged at the same time, the switches 41 are turned OFF as the connected rechargeable battery 42 become fully charged.
In a battery charger of this configuration, when rechargeable batteries are charged with high current for rapid charging in a short time period, charging circuit output current becomes remarkably high. This is because the output current of the charging circuit becomes the sum of the charging currents of all the rechargeable batteries. For example, the output current of a charging circuit to rapidly charge each of four rechargeable batteries with 4 A of current will be a remarkably high current of 16 A.
A battery charger, which charges rechargeable batteries connected in series, has been developed to charge a plurality of rechargeable batteries with a small charging current. However, the charging current of each individual rechargeable battery cannot be controlled when the batteries are connected in series for charging. Consequently, when repeatedly recharged, the rechargeable battery with the most depleted battery capacity will be over-charged and its performance will degrade. Since the charge capacity of the degraded battery will suffer further reduction, that battery will be further over-charged and degrade abruptly.